Manure management on large dairy farms has become a major issue due to air quality and water quality concerns arising from the release of excess nutrients such as nitrogen and phosphorus into the environment. Yet, the nutrients found in manure can be exploited as a valuable fertilizer for any farming operation as it has been used for centuries. The use of manure as fertilizer completes the cycle of nutrients wherein crops feed livestock and the livestock supplies nutrients to the crops.
In spite of the ready availability of manure in dairy operations, use of manure as fertilizer has generally declined on many farms over the past 50 years. This decline in use is due to: 1) farm specialization with increasing separation of crop and livestock production, 2) the cost of transporting manure (which is a bulky, relatively low analysis nutrient source relative to the delivered benefit), and 3) increased availability of high analysis synthetic fertilizers that usually provide a cheaper source per unit of nutrient than manure. Nonetheless, given the bioavailability of the conventionally analyzed nutrients in manure (as well as other nutrients occurring naturally in manure), the mounting demand for produce grown using organic fertilizers in recent years, and the greater awareness of benefits of nutrient conservation in manure-based fertilizer production, dairymen have renewed their interest in the use of manure as or as a component of organic fertilizers.
While manure (raw or composted) contains many nutrients for crop production (including micronutrients), manure is also a valuable source of organic matter for soil amendment. Using manure to increase the volume of organic matter in soil improves the soil's structure or tilth and, thereby: 1) increases the water-holding capacity of coarse-textured sandy soils; 2) improves drainage in fine-textured clay soils; 3) provides a source of slow release nutrients; 4) reduces wind and water erosion; and 5) promotes growth of earthworms and other beneficial soil organisms. Typically, most harvest-type crops suffer the return of only small amounts of crop residue to the soil, so manure, compost, and other organic amendments may be necessary to maintain organic matter levels in soil.
Applying dairy manure in conventional volumes, however, may not yield an ideally balanced nutrient blend in agricultural soils. Often raw dairy manure may not include sufficient levels of nitrogen, phosphate oxide, and potassium oxide (these are the three traditional numbers by which fertilizers are described) to adequately support optimal growth of crops. Where the presence of these three ingredients is too low, the exclusive use of manure to amend soil can lead to nutrient deficiency and, as a result, low crop yields. Properly balanced nutrient blends can enhance manure-based fertilizer to be more efficacious from both a production and environmental standpoint.
On the other hand, where additional synthetic fertilizer is used to enhance the presence of certain nutrients, application of that synthetic fertilizer might result in localized excesses nutrients, which can, in turn, lead to nitrate leaching, phosphorus runoff, accelerated eutrophication of lakes, and excessive vegetative growth of some crops. Thus, understanding how to optimally formulate a manure-based fertilizer is an important element of opportunity for any farming operation with livestock. What is desirable, is the optimal exploitation of manure to serve as a major source of nutrients for growing feed or for resale as agricultural fertilizer.
Unfortunately, the process of composting strips available nitrogen from raw manure; the resulting compost may be very valuable for its organic content though it is deficient of essential nitrogen driven off by the composting process. Fresh, non-composted, manure will generally have a higher nitrogen content than composted manure. So, while uncomposted manure is high in soluble forms of nitrogen, it may contain high numbers of viable weed seeds, which can infest a cultivated crop. In addition, various pathogens such as E. coli may be present in uncomposted manure which can cause illness to individuals eating fresh produce unless proper precautions are taken.
Drying with heat will kill weed seeds and pathogens in the manure. Drying has several benefits in the production of fertilizer. Drying manure to low moisture content reduces the volume and weight of the manure; and that reduction lowers transportation costs. Dried manure products can also be easier to handle and easier to apply uniformly to fields. Dried manure products are readily stored. These advantages are especially prevalent in those dried manures that have been processed and formed into pellets. But, that drying also requires an expensive expenditure of energy; that expense alters the economics of manure use.
The heat necessary to kill pathogens elevates manure temperatures to exceed 150 to 175° F. for at least one hour and ought also to be sufficient to reduce water content to 10 to 12% or less. Composting accomplishes this internal heating necessary to sterilize the manure by exploiting heat generated by microbial respiration. Thus, while either raw manure or composted manure can be sufficiently heated to sterilize it suitable for use as fertilizer, each manure is burdened by its own separate shortcomings. To retain at least some of nitrogen available in raw manure while gaining the benefits of composting the organic material in raw manure, some dairymen blend sterile composted manure with heat-dried uncomposted manure in selected proportions to the retain higher proportions of soluble (inorganic) nitrogen than is present in uncomposted mature, while gaining some of the advantages of stable compost.
Though blended manures will exploit available nitrogen in raw manure to give these blended products some of the desired characteristics. Unfortunately, the amount of heat drying of manure necessary to sterilize the raw manure therein also forces the volatilization of nitrogen in the form of ammonia. The release of ammonia will reduce the total nitrogen content available in the finished product. In addition, composted or partially composted material that has been dried at high temperature rather than going through a curing phase at ambient temperature in a compost process is also not as biologically active as mature compost. Merely dried manure lacks the disease suppressive properties inherent in some composts. This quality of disease suppression depends upon recolonization of the compost by disease suppressing organisms during the curing phase.
As may be apparent, none of these conventional methods of conditioning manure to produce an ideal fertilizer. What is needed in the art is a method and system for recovering nitrogen from manure, isolating compostable organic solids, and to recombine these in selected proportions to retain as well as many of the nutrients present in harvested raw manure. The need exists to recombine nutrients and organic solids in manure into fertilizer while removing water, inert organics and sand, to produce compact and efficacious fertilizer.